1. Field of the Invention
The present invention relates generally to a method of optically recording and reproducing a fine color video signal, and more particularly, a method of optically recording a fine color video signal on a video disc and a method of reproducing the recorded fine color video signal, a video disc on which a fine color video signal is recorded in such a recording manner, and a video disc player for such a video disc.
2. Description of the Background Art
A conventional video disc player for optically reproducing a color video signal of NTSC format optically recorded on a video disc is disclosed in Japanese publication "Laser Disc Technical Book" issued by Corporation ASCII, Nov. 1, 1986.
FIG. 2 is a frequency spectrum diagram showing an FM modulated NTSC color video signal to be recorded on such a conventional video disc. As shown in FIG. 2, the FM modulated video signal comprises an FM deviation area A, an upper side band B and a lower side band C in the range between 4 MHz-13.5 MHz. In the FM deviation area A of FIG. 2, a vertical direction indicates a time axis and a lateral direction indicates a voltage level. The above described FM modulation is performed such that a carrier frequency becomes 7.6 MHz in response to a sync-tip (horizontal synchronizing signal) a of the NTSC color video signal, 8.3 MHz in response to a pedestal level (black level) b, and 9.3 MHz in response to a white peak (white level) c.
On the other hand, a two channel FM audio signal D is recorded on a video disc, which is obtained by respectively FM-modulating a carrier of 2.3 MHz and a carrier of 2.8 MHz by audio signals of different channels.
In addition, a digital audio signal E having the same format as that of a compact disc is recorded on a low frequency side of the above described FM audio signal D. More specifically, two systems of stereo audio signals D and E can be recorded on the video disc, and the contents thereof may be the same or different from each other. In reproducing, either of the stereo audio signals is selected by the video disc player for reproduction.
In recent years, there is more demand for much finer picture reproduced from a video disc. However, with a normal band 4.2 MHz of a NTSC color video signal for broadcasting, sufficient fineness can not be achieved. Now, a method is proposed for forming a fine color video signal including a fine component in a high frequency side by extending a band of a color video signal to be recorded on a video disc to more than the above described 4.2 MHz (for example 5 MHz).
In case of FM-modulating such a fine color video signal for a recording on a video disc, basically, a frequency of an FM modulating carrier should be increased by a frequency corresponding to the extended amount of the above described band, and a linear speed of a recording track during rotation of the video disc should be increased according to a recording band. However, in this case, a recording time of the video disc becomes short, and compatibility with a video disc on which a conventional video signal is recorded is lost.
Now, proposed is a form of a fine color video signal which achieves a maximum compatibility by having a common recording band with that of a conventional color video signal. The video disc on which such form of fine color video signal is recorded and the video disc player for such a video disc are disclosed in Japanese Patent Application No. 63-56942 filed Mar. 10, 1988 by the assignee of the present application (not published yet nor known).
In a video disc produced by such a technique, a fine color video signal with its fine component being preemphasized is FM modulated and recorded with a lower side band of the fine component being multiplexed into a band of a conventional FM audio signal (D of FIG. 2).
Description will be given to a signal processing in reproducing a fine color video signal thus recorded on a video disc, with reference to frequency characteristic diagrams of FIG. 3 to FIG. 5.
Usually in frequency characteristic of an output of a reproduced signal from an optical pick-up of an optical video disc player, a reproduced output of a high frequency is extremely reduced. Accordingly, the frequency characteristics of a fine color video signal reproduced by the optical pick-up from the video disc will be as shown in FIG. 3, wherein the upper side band of the fine component is almost lost. The output having the frequency characteristic shown in FIG. 4 is obtained by passing the pick-up output as shown in FIG. 3 through a limiter amplifier to limit an amplitude thereof. More specifically, although the limiter amplifier functions, as will be described later, so as to reproduce asymmetrical side bands as symmetrical side bands, levels of both the side bands are remarkably lowered. Although the output shown in FIG. 5 will be obtained by FM demodulating the output of the limiter amplifier shown in FIG. 4, the high frequency level of the FM demodulated output will be lowered by the amount of a drop of the level of the upper side band of the limiter amplifier output as shown in FIG. 4. Compensation of a drop of the high frequency component of the FM demodulated output to flatten the frequency characteristic after FM demodulation leads an amplification of the noise component in the high frequency area, so that an S/N ratio is deteriorated as much.
Relation between such a drop of the side band level of the FM wave and deterioration of the frequency characteristic after FM demodulation will be theoretically explained furthermore with reference to FIG. 6 and FIG. 7.
FIG. 6 and FIGS. 7(a)-(c) are vector diagrams of an FM wave. In general, when a modulation index of the FM wave is small, a side band of second or higher order is ignorable. In FIG. 6, OA is a vector of a carrier, while AB and AC are vectors of the upper side band and the lower side band, respectively. A vector OD which is a composite of all these vectors will be a vector of FM wave.
Since both the side band vectors rotate in directions opposite to each other as shown in FIG. 6, at a modulating frequency, the composite vector AD of both the side bands moves to and from on a line perpendicular to a carrier vector OA. The composite vector of the side band vectors represents a movement toward the carrier frequency, that is, the frequency modulation. More specifically, the magnitude of this vector represents a frequency deviation of the FM wave. If a level of one of the side bands is lowered, for example, if one of them disappears, a locus of the composite vector of the side bands draws a circle as shown in FIG. 7(b). As a result, the FM wave includes fluctuation in the direction of amplitude, that is, an amplitude modulation (AM) component in addition to fluctuation in the direction of frequency. By passing a signal having such an AM component through a limiter amplifier, the fluctuation in the direction of amplitude is suppressed, so that the composite vector of the side band vectors starts moving on the line perpendicular to the carrier vector OA as shown in FIG. 7(c). However, the magnitude, that is, the frequency deviation is half of that of the original FM wave. As shown in FIG. 7(c), FM demodulation of the FM wave having half the frequency deviation results in a lowered demodulation output level by half.
In addition, when the level of the one side band is lowered but does not disappear, a locus of the composite vector of both the side band vectors draws oval, and the frequency deviation of the FM wave becomes smaller similar to the above described case.
As the foregoing, since the upper side band of the fine component remarkably attenuates or disappears by a passing of the FM wave through the optical pick-up having such frequency characteristic as shown in FIG. 3, the high frequency output of the fine color video signal after FM demodulation is reduced, so that a sufficiently fine picture could not be obtained.